1. Field of the Invention
The present invention relates to a distance-settable photoelectric sensor that is based on the principle of triangulation and in which an optical system is constituted by mounting a condenser lens to a light projecting element.
2. Description of the Related Art
Conventionally, distance-settable photoelectric sensors that are based on the principle of triangulation are known.
First, the principle of triangulation, which is employed in a typical distance-settable photoelectric sensor, and the mechanism for using projected light effectively by mounting a condenser lens to a distance-settable photoelectric sensor will be described with reference to FIGS. 1A and 1B.
As illustrated in FIG. 1B, in a distance-settable photoelectric sensor, for example, light emitted by a light projecting element 101, such as a light projecting LED, that serves as a light emitting source is projected onto a detection region by a light projecting lens 102 to irradiate the surface of an object 20. Then, (a portion of) the light reflected and diffused by the surface of the object 20 is condensed by a light receiving lens 103 and received by a light receiving element 104.
The light receiving element 104 is, for example, a light receiving element, such as a multi-segment photodiode, and receives the light reflected and diffused by the surface of the object 20 at a position corresponding to the distance in each of the cases in which the object 20 is at a distance A of a relatively small distance and in which the object 20 is at a distance B of a relatively large distance, as illustrated in FIG. 1B. In other words, the light receiving position on the light receiving element 104 changes in accordance with the distance to the object 20.
Therefore, the distance to the object 20 can be measured by detecting the light receiving position on the light receiving element 104.
Meanwhile, with regard to photoelectric sensors, conventionally, a technique in which a spherical condenser lens is mounted to a light emitting element or a light receiving element to constitute an optical system is disclosed (e.g., Japanese Unexamined Patent Application Publication No. 4-13989).
Affixing the condenser lens makes it possible to increase the quantity of light by condensing the projected light. Thus, this technique is employed in a distance-settable photoelectric sensor illustrated in FIG. 1A, and a spherical condenser lens 105 is mounted to the light projecting element 101 illustrated in FIG. 1B so as to effectively use the light from the light projecting element 101.
FIG. 2 is an illustration for describing the effect of the condenser lens.
As illustrated in FIG. 2, mounting the condenser lens 105 to the light projecting element 101 makes it possible to effectively use the projected light, and the light projection power greatly improves.
FIG. 3A illustrates a change in the light receiving range on the light receiving element 104 in a case in which the spherical condenser lens 105 is mounted to the light projecting element 101 in a photoelectric sensor such as the one illustrated in FIGS. 1A and 1B, for example. A portion indicated by a in FIG. 3A is the light receiving range on the light receiving element 104 in the case in which the object 20 is at the distance A, and a portion indicated by b is the light receiving range on the light receiving element 104 in the case in which the object 20 is at the distance B. In addition, a portion indicated by c in FIG. 3A is a portion in which the light receiving range a and the light receiving range b overlap.
When the spherical condenser lens 105 is mounted to the light projecting element 101, the light receiving range increases, as illustrated in FIG. 3A.
In other words, when the spherical condenser lens 105 is mounted to the light projecting element 101, the width of the light receiving distribution on the light receiving element 104 increases both in the X-direction and in the Y-direction, or in other words, increases both in the lateral direction and in the longitudinal direction along the surface that receives the light that has passed through the light receiving lens 103.
In contrast, FIG. 3B illustrates a change in the light receiving range on a light receiving element in a case in which the condenser lens 105 as illustrated in FIG. 1A is not mounted to a light emitting element and a light receiving element in a typical distance-settable photoelectric sensor, or in other words, illustrates a change in the light receiving range on the light receiving element illustrated in FIG. 1B.
As compared to the light receiving range illustrated in FIG. 3A, the light receiving range on the light receiving element is smaller in FIG. 3B than in FIG. 3A (refer to a′ and b′ in FIG. 3B).
However, if the light receiving width in the Y-direction increases as the condenser lens 105 is mounted, there is a problem in that, when the change in the position of the object 20 is small, for example, this change in the position of the object 20 may not be detected with accuracy.
Specifically, when the position of the object 20 changes from the distance B to the distance A, the light receiving range on the light receiving element 104 also changes from b to a, as illustrated in FIG. 3A. However, since b and a overlap in the range indicated by c in FIG. 3A, the amount of change in the light receiving signal on the light receiving element 104 is small, which makes it hard to determine that the distance to the object 20 has changed.